1. Field of the Invention
The present invention relates to a disconnection detecting circuit which judges whether there is a disconnection or not in a signal line connecting a micro processor and an IC or a sensor which are used with high reliability at the same time in environments accompanied by vibration, such as in planes and automobiles.
2. Description of Related Art
FIG. 10 is an explanatory diagram to show one example of a conventional disconnection detecting circuit. Reference numeral 1 denotes a device having a disconnection detecting circuit, such as a micro processor, reference sign 1a denotes a body of the device 1, i.e., IC package, reference numeral 2 denotes a lead member provided in the IC package 1a, reference numeral 3 denotes an IC chip on which an integrated circuit is formed, reference numeral 4 denotes a bonding wire for connecting the lead member 2 and the IC chip 3, reference numeral 5 denotes a bonding pad connected to the lead member 2 with the bonding wire 4, reference numeral 6 denotes an output buffer which is a constituent of the integrated circuit formed on the IC chip 3, reference numeral 7 denotes an input buffer which is a constituent of the integrated circuit formed on the IC chip 3, reference numeral 8 denotes a peripheral device such as an integrated circuit device (hereinafter referred to as xe2x80x9cICxe2x80x9d) or a sensor, which is connected to the device 1 and reference numeral 9 denotes a signal line for connecting the device 1 and the peripheral device 8.
Next, an operation of the disconnection detecting circuit will be discussed.
Discussion will be made on a disconnection detecting circuit in which the device 1 and the peripheral device 8 are connected with the signal line 9 as an exemplary conventional one. In advance a predetermined signal or signal level is set in the peripheral device 8 as an expected value. The peripheral device 8 outputs the expected value signal to the signal line 9 like an ordinary transmit data, to transmit it to the device 1. The device 1 judges whether the received expected value signal coincides with the signal or signal level set in advance or not, for judgment on whether there is a disconnection or not in the signal line 9.
FIG. 11 is an explanatory diagram to show another example of conventional disconnection detecting circuit. The disconnection detecting circuit shown in FIG. 11 is intended for judgment on whether there is a disconnection or not in the signal line 9 without outputting the expected value signal from the peripheral device 8, and constituent elements thereof identical to or corresponding to those of the disconnection detecting circuit of FIG. 10 are denoted by the same reference signs and further description thereon will be omitted. In FIG. 11, reference numeral 10 denotes a resistor loaded on the signal line 9 and reference numeral 11 denotes wiring capacitance generated in the signal line 9 which connects the peripheral device 8 and the device 1.
Next, an operation of the disconnection detecting circuit will be discussed.
The disconnection detecting circuit shown in FIG. 11 detects disconnection by accumulating electric charges in the following procedure. To begin with, a predetermined signal is temporarily output from the output buffer 6 of the device 1 to the signal line 9. Then, the electric charges are accumulated in the wiring capacitance 11 of the signal line 9. After that, the output of the output buffer 6 is stopped and a given period later, the signal input to the device 1 through the lead member 2 is observed. From this observation result, it is judged whether there is a disconnection or not as follows. The observation is made on the output side of the input buffer 7.
1. When a signal of high level (hereinafter referred to as xe2x80x9cHxe2x80x9d) is temporarily output from the output buffer 6 and H electric charges are accumulated in the wiring capacitance 11, judgment is made as follows:
i) If the output of the input buffer is H, there is a possibility of disconnection.
ii) If the output of the input buffer is low level (hereinafter referred to as xe2x80x9cLxe2x80x9d), there is no possibility of disconnection.
This judgment on whether there is a disconnection or not, is made from a fact that if a terminal voltage of the peripheral device 8 connected to the signal line 9 is L, when an H signal is output from the output buffer 6 of the device 1 to the signal line 9 for a given period and stopped, the potential of the signal line 9 falls from H level and converges on the terminal voltage of the peripheral device 8.
2. When an L signal is temporarily output from the output buffer 6 and L electric charges are accumulated in the wiring capacitance 11, judgment is made as follows:
i) If the output of the input buffer is H, there is no possibility of disconnection.
ii) If the output of the input buffer is L, there is a possibility of disconnection.
This judgment on whether there is a disconnection or not, is made from a fact that if the terminal voltage of the peripheral device 8 connected to the signal line 9 is H, when an L signal is output from the output buffer 6 of the device 1 to the signal line 9 for a given period and stopped, the potential of the signal line 9 rises from L level and converges on the terminal voltage of the peripheral device 8.
When it can be judged that there is a possibility of disconnection in both cases of the above items 1 and 2, it is supposed that disconnection occurs between the resistor 10 and the device 1. Further, when it is judged that there is no possibility of disconnection in either case of the above item 1 or 2, it can be judged that disconnection does not occur between the resistor 10 and the device 1.
That is, if the electric charges output from the output buffer 6 are accumulated in the wiring capacitance 11 for a longer time than a predetermined period, it is judged that disconnection occurs in the signal line 9 and if the electric charges accumulated in the wiring capacitance 11 are not detected at the output of the input buffer 7 by a drive of the peripheral device 8 connected to the device 1 through the resistor 10 in either item 1 or 2, it is judged that the signal line is normally connected. Further, when the output of the input buffer 7 is read, the output of the output buffer 6 should be brought into a high impedance state so that it does not affect data to be measured which are output from the peripheral device 8.
Since the conventional disconnection detecting circuit has the above constitution, it is possible to perform a relatively easy detection of disconnection in the signal line or the like when the peripheral device has a function of outputting the expected value signal set in advance. However, when the peripheral device has no function of outputting the expected value signal, the resistor is inserted in the signal line as shown in FIG. 11 and it is indirectly judged whether disconnection occurs in the signal line, using the characteristics that the wiring capacitance holds the electric charges for a given period. In this case, the time period while the electric charges are accumulated in the wiring capacitance and the length of time until the electric charges accumulated in the wiring capacitance are observed are closely related to a lot of factors, such as the resistance value of the resistor, the output characteristics of the peripheral device, the value of the wiring capacitance and the characteristics of the output buffer and the input buffer provided in the device, and they can not be uniquely determined. This causes a problem that the resistance value of the resistor, the output time of the output buffer and the length of time until the electric charges accumulated in the wiring capacitance are observed should be determined, with repeated trial and error after the circuit employing the device and the peripheral device are completed.
The present invention is intended to solve the above described problems and it is an object of the present invention to provide a disconnection detecting circuit capable of detecting disconnection in a signal line by a static measurement without inserting electronic components such as a resistor or the like in the signal line or without relation to an amount of the wiring capacitance, the resistance value of wires, the output characteristics of the peripheral device or the characteristics of the output buffer provided in the device even if the peripheral device has no function of outputting the expected value signal.
It is another object of the present invention is to provide a disconnection detecting circuit capable of judging whether there is a disconnection or not in a wire between the peripheral device and a lead member serving as an input terminal of the device and, in addition, in a bonding wire connecting between a bonding pad to the lead member in the device and whether there is a poor connection or not in the connecting portion thereof.
A disconnection detecting circuit according to the present invention includes a switch for setting a detection signal to be supplied for the signal line, a resistor for supplying the signal line with the detection signal set by the switch, and control means for controlling the switch to set the detection signal to be supplied for the signal line and judging whether there is a disconnection or not in the signal line on the basis of a change in the detection signal, which are provided in the device.
Therefore, the disconnection detecting circuit according to the present invention can produce an effect of detecting disconnection in a short time with a simple constitution.